In recent years, demand for high-strength steel sheet with a good workability designed for improvement of the fuel efficiency and improvement of the durability of automobile frames and members has been rising. In addition, steel sheet of a tensile strength of the 780 MPa class or more is being used for frame parts or reinforcement or other members from the need for collision safety and expanded cabin space.
The first important thing with steel sheet for a frame is its spot weldability. Frame parts absorb impact at the time of collision and thereby function to protect the passengers. If a spot weld zone is not sufficient in strength, it will break at the time of collision and sufficient collision energy absorption performance will not be able to be obtained.
Technology regarding high-strength steel sheet considering weldability is, for example, disclosed in Japanese Patent Publication (A) No. 2003-193194 and Japanese Patent Publication (A) No. 2000-80440. Further, weldability is also studied in Japanese Patent Publication (A) No. 57-110650, but this only discusses flush butt weldability and does not disclose anything regarding technology for improving the spot weldability important in the present invention.
Next, a high yield strength is important. That is, a high yield ratio material is superior in collision energy absorption ability. To obtain a high yield ratio, making the structure a bainite structure is useful. Japanese Patent Publication (A) No. 2001-355043 discloses steel sheet having a bainite structure as a main phase and a method of production of the same.
Finally, the workability of the steel sheet, that is, the ductility, bendability, stretch flange formability, etc. are important. For example, “CAMP-ISIJ vol. 13 (2000) p. 395” discloses, regarding hole-expandability, that making the main phase bainite improves the hole-expandability and, regarding the punch stretch formability, that forming residual austenite in a second phase results in a punch stretchability on a par with current residual austenite steel.
Further, it discloses that if performing austempering at the Ms temperature or less to form 2 to 3 vol % residual austenite, the tensile strength×hole-expandability becomes maximum.
Further, to increase the ductility of high-strength materials, the general practice is to make positive use of a composite structure.
However, when using martensite or residual austenite as a second phase, the hole-expandability ends up remarkably dropping. This problem is for example disclosed in “CAMP-ISIJ vol. 13 (2000), p. 391”.
Further, the above document discloses that if making the main phase ferrite, making the second phase martensite, and reducing the difference in hardness between the two, the hole-expandability is improved. Further, an example of steel sheet superior in hole-expandability and ductility is disclosed in Japanese Patent Publication (A) No. 2001-355043.
However, steel sheet having a tensile strength of 780 MPa or more provided with a high yield ratio and good ductility and further good in spot weldability cannot be said to have been sufficiently studied.
In particular, regarding spot weldability, with high-strength steel sheet, rather the weld zone strength falls. If welding by a welding current of the expulsion and surface flash region, the weld zone strength will remarkably drop or fluctuate. This problem is becoming a factor blocking expansion of the high-strength steel sheet market.